Bone and Soft-Tissue Disorders
= INFUNDIBULAR CYST = SEBACEOUS CYST (misnomer)
= proliferation of surface epidermal cells within a circumscribed space in the dermis
Histo: production of keratin within closed space lined by surface epidermis
Associated with: nevoid basal cell syndrome (Gorlin syndrome) + high prevalence of epidermoid / dermoid cysts
- intradermal / subcutaneous mass
Location: anywhere; subungual (common)
- small unilocular / (rarely) large multiloculated mass
US:
- circumscribed circular / oval hypoechoic mass on US
- often associated with hair follicle
MR:
- isointense / slightly hypointense relative to muscle on T1WI
- hyperintense with focal areas of decreased signal on T2WI
- no appreciable enhancement
Epidermal Inclusion Cyst
= INTRAOSSEOUS KERATIN CYST = IMPLANTATION CYST
Age: 2nd–4th decade; M >F
Cause: trauma / migration of nail bed fragment → entrapment of epidermal fragments within other tissues
Path: encapsulated round / oval lesion lined by stratified squamous epithelium + filled with laminated keratin (soft white cheesy contents)
Histo: stratified squamous cells, keratin, cholesterol crystals
- history of trauma (implantation of epithelium under skin → secondary bone erosion); asymptomatic
- swelling of fingertip with redness, pain, sensation of heat
Location: superficially situated bones such as calvarium (typically in frontal / parietal bone), phalanx (usually terminal tuft of middle finger), L >R hand, occasionally in foot
- well-defined round osteolysis with sclerotic margin
- cortex frequently expanded + thinned
- NO calcifications / periosteal reaction / soft-tissue swelling
- ± wall calcification / ossification
- pathologic fracture often without periosteal reaction
US:
- round / oval an- or hypoechoic subungual mass with edge shadowing containing variably echogenic foci
- NO internal vascularity
MR:
- well-defined round lesion of intermediate T1 + T2 SI
- central debris + thin peripheral rim of enhancement
- scalloping of distal phalanx underlying nail bed
- extensive inflammation of surrounding soft tissue ← suggestive of lesion rupture
DDx:
- in finger: glomus tumor, enchondroma (rare in terminal phalanx)
- in skull: infection, metastasis (poorly defined), eosinophilic granuloma (beveled margin)